A microscope objective and electronic imaging system were used to determine how isolated frog skeletal muscle fibers adjust their volume during an isometric tetanus. Cross-sectional area and volume of the middle third of a fiber increased rapidly with the development of active tension, which indicates that contraction produced components of force perpendicular to the long axis. The extreme ends are known to shorten whether or not the middle of a fiber is isometric or stretched. Shortening of the ends may shift water towards the middle, which could account for the volume changes we observed. The cytoskeletal matrices of muscle evidently adjust rapidly during contraction to maintain a dynamic equilibrium between the axial and radial forces that stabilize the whole cell. The Z disks have been shown to expand during active, but not passive, tension development. Z disks might be the elastic elements of the muscle cytoskeleton primarily involved in rapid balancing of the radial components of active force.